Refinery cracking processes serve to upgrade the heavier portions of petroleum to large volumes of lighter, more valuable hydrocarbon products. The cracking process is performed catalytically or thermally, the particular method depending on the type of petroleum feedstock being processed. The older thermal cracking processes, which include delayed cokers and flexicokers, is used to process heavy petroleum fractions, while the newer catalytic cracking process, which has developed into one of the most important petroleum refining processes, is used to crack lighter petroleum fractions, such as vacuum gas oil. The dominant catalytic cracking process in use today is the fluid catalytic cracking (FCC) process.
In FCC processes a mixture of hydrocarbon feedstock and steam is injected into a section of a hydrocarbon reactor unit referred to as the riser, where it contacts hot regenerated catalyst. Operating temperatures in the riser-reactor are typically in the range of about 450.degree. to about 575.degree. C. Cracking reactions begin immediately, producing an array of lower boiling hydrocarbons. The catalyst and cracked hydrocarbon vapors are carried up the riser into the catalyst-vapor disengagement section. Hydrocarbon condensation reactions also occur in the riser, with the result that coke is produced and deposited on the active sites of the catalyst. This substantially reduces the catalyst activity and selectivity.
The cracked hydrocarbon vapors and coked catalyst are separated in the reactor disengagement section. The separated catalyst drops by gravity into the stripping section of the unit, where hydrocarbons entrained with the catalyst are separated therefrom by means of stripping steam. The catalyst-free hydrocarbon vapors leave the reactor unit through the cracked product transfer line, and are conveyed to the main column for fractionation.
The cracked hydrocarbons enter the main column, where they are separated into one or more liquid streams and an overhead vapor stream. The overhead vapor stream, which consists of light gasoline and C.sub.4 and lighter hydrocarbons, hydrogen and perhaps light inert gases, such as nitrogen, is cooled and discharged into the overhead accumulator vessel, where it undergoes flash separation to yield two hydrocarbon streams: a vapor stream, comprised substantially of C.sub.4 and lighter hydrocarbons and hydrogen, and a liquid stream comprised of C.sub.5 and heavier hydrocarbons, typically called unstabilized gasoline.
The overhead accumulator vapor stream, typically referred to as wet gas, is subsequently compressed for downstream fractionation steps. The compression is usually conducted in a train of compressors comprising two stages of compression, with interstage condensation and removal of additional unstabilized gasoline. The compressed stream is sent to a high pressure receiver vessel, from which a gaseous stream and a liquid stream are separated. The gaseous stream, referred to as high pressure gas and comprised mostly of C.sub.2.sup.- hydrocarbons and hydrogen, is sent to a series of absorbers and distillation columns for recovery of the various components of this stream. The liquid stream from the high pressure receiver is likewise subjected to downstream processing steps for recovery of its components.
The stripped catalyst from the disengagement section of the cracking reactor flows into the catalyst regenerator. A controlled amount of air is blown into this vessel to rejuvenate the catalyst by combusting the coke on the catalyst, which is maintained in a fluidized state in the regenerator. The coke combustion reactions are highly exothermic accordingly the catalyst becomes very hot, e.g. its temperature after regeneration is generally in the range of about 560.degree. to about 800.degree. C. Regenerated catalyst is carried out of the regenerator through the regenerated catalyst standpipe and is introduced into the reactor riser, thereby completing the catalyst cycle.
The rate of catalyst flow to the riser is typically controlled by a slide-valve in the regenerated catalyst standpipe. Steady flow of catalyst through this valve is maintained by maintaining a steady residual pressure drop across the valve. For this purpose, it is essential that the regenerator pressure be maintained at a pressure higher than the reactor vessel pressure. Thus, the minimum pressure in the catalyst regenerator can be determined by the pressure in the cracking reactor.
The demand for refined hydrocarbon products has increased the incentive to maximize the amount of throughput or conversion in refinery FCC systems. Operation at higher FCC throughput or conversion increases wet-gas production, which in turn, increases production of valuable light hydrocarbons. The ability to increase FCC hydrocarbon throughput or conversion is very often limited by one or more of: (i) wet-gas compression capacity, (ii) regenerator coke burning capacity, and (iii) the ability to circulate catalyst by maintenance of required pressure drop across critical elements of the system.
Wet-gas compressor throughput can be enhanced by operating with a higher compressor inlet suction pressure, which can be attained by increasing the overhead accumulator pressure. For instance, increasing the overhead accumulator pressure by 1 psi can increase wet-gas compressor capacity by up to about 4%. However, the pressure in the overhead accumulator controls the upstream pressure, i.e. the pressure in the main column and the hydrocarbon riser-reactor. Increasing the overhead accumulator pressure would cause an increase in riser-reactor pressure, which is undesirable from a cracking perspective, because higher cracking reaction pressures enhance the selectivity of the coke-forming condensation reaction at the, expense of the desirable cracking reactions. Furthermore, the riser-reactor can encounter a catalyst circulation limit and flow reversal, unless the catalyst regenerator pressure is also correspondingly increased. Increasing the regenerator pressure is undesirable because this increases the air blower discharge pressure, which reduces its output.
Wet-gas compressor throughput can also be increased by lowering the compressor discharge pressure. This is likewise undesirable because the corresponding lower deethanizer-absorber pressure causes propylene to be lost to fuel gas. Quantitatively, lowering the absorber pressure by 20 psi could increase gas compressor throughput by 5%, but it also results in a 1.5% reduction in propylene recovery.
In addition to increasing wet-gas production, operation at higher throughput or conversion increases coke production rate, which tends to push units to their limiting ability to regenerate spent catalyst. The actual limit can be due to the limiting amount of air which the air blower can discharge. Some relief from an air-blower limit may be obtained by lowering the regenerator pressure, which enables the blower to discharge more moles of oxygen (in the form of air or oxygen plus inert diluent). The increased amount of oxygen availability allows the combustion of a larger amount of coke, which also releases more heat energy in the regenerator. The increased heat release and coke burning capacity can be taken advantage of by increasing the feed rate to the riser-reactor, and allowing the unit to establish a different heat and coke balance. However, lowering the regenerator pressure at constant riser pressure, could also lead to a catalyst circulation limit and flow reversal. A catalyst circulation limit may also be directly encountered due to the higher catalyst circulation rate required to process higher throughput at constant conversion.. From the above discussion, it is clear that the major constraints on an FCC unit are all strongly interrelated, and operational moves made to obtain relief from any one constraint may push the unit towards another constraint.
The load on the wet-gas compressor system would be considerably lessened if the light components could be removed from the wet-gas. This would permit a reduction in overhead accumulator pressure, which would, in turn, increase hydrocarbon throughput and conversion in the cracking reactor. It would be very advantageous to reduce the volume of wet gas that must be handled by the wet gas compressors by diverting .all or a portion of the compressor feed gas to an adsorption system comprised of one or more adsorption beds containing adsorbent(s) that adsorb C.sub.2 and higher hydrocarbons more strongly than they adsorb methane and hydrogen. Methane and hydrogen and any other light inert gases that are present in the wet gas, such as nitrogen and argon, can then pass through the adsorption beds as nonadsorbed component and be sent to refinery fuel, or otherwise disposed of, while the adsorbed gas component, comprised of hydrocarbons higher than methane is desorbed from the adsorbent and sent to the wet gas compressors, where it is compressed and sent to downstream hydrocarbon separation units.
It is known to burn coke off FCC spent catalyst in catalyst regenerator by contacting the catalyst with a mixture of oxygen and carbon dioxide. It would be very advantageous to gradually replace the air in air-operated catalyst regenerators with oxygen-carbon dioxide mixture by passing the regenerator exhaust gas, which is rich in nitrogen and carbon dioxide, through a pressure swing adsorption system to remove the nitrogen, and then introduce the carbon dioxide to the regenerator with oxygen. This procedure would only require the use of the adsorption system for a short while, and unless another use were found for the adsorption system, it would sit idle until the next time it is needed for this purpose, i.e. after the next FCC plant turnaround. It would be very advantageous to use the adsorption system in a manner that would enhance the overall efficiency of the FCC operation. This invention provides a means of achieving this, and also enables the operator to make operational moves that would debottleneck other constraints on the unit.